COMPARISON OF SM-ND AND LU-HF GARNET AGES I: GRANULITE METAMORPHISM, FIORDLAND NEW ZEALAND

Garnet Sm-Nd & Lu-Hf ages constrain the timing of garnet growth; however, the 2 systems seldom yield the same results. In low T rocks, age discrepancies may result from zoning of parent atoms in garnet and age bias toward initial growth. In high T rocks, age discrepancies are generally attributed to diffusion which resets isotopic systems. The effects of diffusion will depend on T, diffusion constants, cooling rate, grain size/diffusion length scales, and relative diffusion of parent and daughter. The Western Fiordland Orthogneiss is ideal for evaluating these processes because it contains a wide range of garnet sizes and experienced a relatively simple thermal history which peaked at >900°C & 14 kbar.

Samples with garnet ranging from 1 to 0.1 cm diameter were selected to compare ages for ca. 1 cm grains using both systems and to compare Sm-Nd ages for 1.0 to 0.1 cm grains. All garnet grains display weak major element zoning with broad unzoned cores and <1 mm zoned rims. In ca. 1 cm garnet grains, zoning includes high-magnitude, sharp, oscillations in Lu and Hf. Sm is weakly zoned with broad low-amplitude bell-shaped profiles in Sm. We conclude that Lu, Hf, and likely Sm did not diffuse significantly in large garnet. In the 1 to 5 mm garnet, trace element data are not available, but major element data are compatible with diffusional re-equilibration.

Lu-Hf ages for ca. 1 cm garnets in 2 samples (garnet only isochrons) are 114.8±3.5 (N=7; MSWD=0.2) & 116.8±1.8 Ma (N=7; MSWD=1.3) and Sm-Nd ages from the same samples are 113.1±2.3 (N=10; MSWD=2.1) & 114.9±3.5 Ma (N=8; MSWD=2.1). Small garnets in 3 samples have Sm-Nd ages of 103.4±2.3 (1.25 mm; N=5; MSWD=2.1), 108.1±1.8 (2.5 mm; N=5; MSWD=1.0), and 104.4±2.0 Ma (1.5 mm; N=5; MSWD=1.5). Estimated diffusion coefficients of Lu, Hf, Sm, and Nd in garnet have significant ranges and partitioning with adjacent minerals is likely important. However, closure T for the ca. 1 cm garnet grains are >>1,100°C for Lu-Hf and 860-1060°C for Sm-Nd. The Sm-Nd garnet ages decline with grain size compatible with diffusion-controlled ages during cooling from peak conditions. Using the Carlson (2012) Sm and Nd diffusion data, closure T are 865-1065°C with cooling at -15 C/m.y. We conclude that Sm-Nd data for 1.0-0.1 cm garnet grains track the timing of metamorphism from the peak of granulite T to ca. 850°C.